Cathode ray display tube

ABSTRACT

A cathode ray tube for a high frequency oscilloscope includes a flood gun disposed within the tube to excite the phosphor material in the display screen for improved electron beam writing rates and for postfogging photographic film used to record signal traces written on the display screen. Flood gun control circuitry activates the flood gun intermittently in timed relationship to the operation of the electron beam which produces the signal trace on the display screen.

United States Patent James R. Pettit;

David E. Chaffee; James R. Ashley, Colorado Springs, Colo.

Feb. 24, 1969 Mar. 9, 1971 Hewlett-Packard Company Palo Alto, Calif.

Inventors Appl. No. Filed Patented Assignee CATI-IODE RAY DISPLAY TUBE 5 Claims, 3 Drawing Figs.

US. Cl

Int. Cl H01j 29/50 Field ofSearch 315/10, 13

References Cited UNITED STATES PATENTS 7/1968 Kruger Primary Examiner-Rodney D. Bennett, Jr. Assistant ExaminerJoseph G. Baxter Attorney-A. C. Smith ABSTRACT: A cathode ray tube for a high frequency oscilloscope includes a flood gun disposed within the tube to excite the phosphor material in the display screen for improved electron beam writing rates and for postfogging photographic film used to record signal traces written on the display screen. Flood gun control circuitry activates the flood gun intermittently in timed relationship to the operation of the electron beam which produces the signal trace on the display screen.

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' SHEET 2 OF 2 INVENTORS JAMES R. PETTIT DAVID E. CHAFFEE JAMES R. ASHLEY a-c- Smixk ATTORNEY (IAT HODE RAY MSPLAY TUBE BACKGROUND OF THE INVENTION Cathode ray display tubes for displaying signal frequencies above llllll megahertz typically produce only low intensity Inminous traces on the display screen because the extremely short period of phosphor excitation by the electron beam per unit area of the display screen is inadequate for exciting the phosphor material to an acceptable luminous intensity.

in the past, writing rate was increased by reducing the area of the display screen typically to less than 10 square centimeters and this increased the phosphor excitation time per unit area to provide a signal trace of acceptable luminous intensity. Small display screens of this type, however, usually have the concomitant disadvantage of providing poor signal detail and resolution. Also, such small display screens are inconvenient to use since the signal trace can usually be observed only through an eyepiece viewer.

SUMMARY OF THE INVENTION Accordingly, the cathode ray display tube of the present invention includes a writing electron-beam gun and a flood electron gun disposed within the tube to excite the phosphor material in a display screen which has an area of cover 60 square centimeters. The flood gun and writing gun are operated in a selected timed sequence to provide postexcitation for the phosphor material in the display screen. Also, the flood gun is designed to provide a substantially uniform flood of electrons over the area of the display screen from a location within the tube which is remote from the central axis of the tube.

DESCRIPTION OF THE DRAWINGS FIG. 3 is a pictorial view of the cathode ray display tube and control circuitry according to one embodiment of the present invention;

ElGS. 2a through e are graphs of waveforms as a function of time in the circuit of HG. l; and

HO. 3 is a sectional view of the flood electron gun in the tube shown in HQ. l.

DESCRlPTlON OF THE PREFERRED EMBODIMENT Referring now to FIG. ll, there is shown a cathode ray display tube 9 including a writing electron beam gun ll and a singie flood electron gun 13 disposed within an evacuated envelope lid. The display screen l7 end of the envelope l is arranged symmetrically about and normal to the central axis 19 of the writing gun ll and includes an inner surface coating 21 of luminous phosphor material including zinc sulfide and copper, commonly classified as P-l 1 or P-3l phosphor.

The writing gun ll may include such conventional electrodes as the cathode 23, focusing and accelerating electrodes (not shown), beam-blanking grid 25, vertical deflection plates 2d, 27, horizontal deflection plates 2%, 29, spherical reference mesh for the postaccelerator region 33, and the like. The postaccelerator electrode 35 may be conventionally formed as a continuous conductive layer or as a spiral strip on the inner or outer surface of the envelope 15. The flood gun 13 may be disposed within the postaccelerator region 33 ahead of the spherical mesh electrode Ell as in a storage-type cathode ray tube. However, for cathode ray tube operation at extremely high writing rates, it is desirable in the present invention to position the flood gun l3 behind (i.e. on the deflection side of) the mesh 311. The flood gun E3 is positioned to one side of the central axis ill for supplying electrons substantially undorrnly over the area of the display screen 17. lBiasing 37 are provided for supplying operating signals to electrodes of the tube through connections which have been omitted from the drawing for clarity and simplicity. in addition, certain of the electrodes of the tube 9 receive time-varying signals, as shown in the graph of FIG. 2, during operation to display the waveform of an applied input signal.

An input signal is applied to trigger circuit 3? which may be responsive to a selectable level or slope, or the like, for producing an output on line 39. Output signal on line 39 is applied to the sweep generator 41 which in turn applies a ramp or sweep signal to the horizontal deflection plates 28 and 29 during time t, and as shown in FlG. 2a. Also, the signal on line 39 is applied during time t, to to the unblanlting circuit 44 connected to blanking grid 25, as shown in HO. 2%, for permitting the writing beam to impinge upon the display screen 17. At the same time, the input signal is applied through delay means 43 and amplifier 445 to the vertical deflection plates 26, 27, as shown in FlG. Be. in this manner, the sweep signal is initiated and the waveform of the input signal is traced out in a conventional manner during the time t to by the deflection of the electron beam about the central axis ill in response to the time-varying signals applied to the horizontal and vertical deflection plates 26-49.

At very high frequencies, the rapid deflection of the writing beam over the area of the display screen 17 results in low concentration of electron beam power per unit time in a given beam target area, thus producing a signal trace on the display screen of low luminous intensity. The luminous intensity of the phosphor material, say, at a location on the display screen near the beginning of the sweep decays with time, as shown in H6. 20, to about 50 percent intensity in approximately l0-" seconds for P-ll phosphor and in about 4 X 10- seconds for P-3l phosphor.

The luminous intensity of the signal trace may be increased according to the present invention by supplying flood electrons to the display screen in timeshared relationship to the writing beam, as shown in FIG. 2d. The flood gun 13 of FIG. 1

.is activated by the flood gun control 47 connected thereto to supply a flood of electrons to the display screen 17 after the writing beam has been blanked at the end of a sweep t These combined excitations of the phosphor material in the coating 21 of the display screen 17 produce substantially higher intensity luminous output than is possible without the added flood electrons. it is believed that this effect may be properly described as follows: As the radiative processes of phosphor materials are presently understood, it appears that activator centers (i.e. imperfections, impurities, etc.) in the crystal of the phosphor material can play dominant roles in trapping excited electrons and positive holes and in converting excitation energy into phonons (i.e. crystal vibrations) as well as photons. Excitation energy may be transferred to and between these centers which are dispersed throughout the crystal structure of the phosphor material either directly or by such indirect means as excited electrons, positive hole migrations, resonant transfers, collisions, photons, 'phonons, and the like. Thus excitation of the phosphor material may raise electrons from occupied energy levels to unoccupied levels when the energy separation between the two levels is less than the excitation energy. It is believed that these activator centers in troduce many additional allowed energy levels that may be occupied within the otherwise forbidden energy band gap (i.e. between conduction and valence bands) of the host-crystal phosphor material. Thus an excited electron in the conduction band of the host crystal tends to lose energy rapidly to the crystal by interaction with phonons and by other processes until the energy of the electron has been reduced so that it occupies a level near the lower edge of the conduction band. At this point, the excited electron has relatively little lrinetic energy and so it may become temporarily trapped by dropping into an unoccupied additional level introduced by an activator center, giving up excess energy as phonons. Electrons in these traps may be raised in energy level into the conduction band again by supplying sufiicient excitation energy. This increases the probability of radiative transitions occurring as electrons fall in energy from a level near the lower edge of the conduction band.

it is believed that these radiative transitions between two energy levels (the energy difference being emitted as a photon) may occur in several ways. For example, where the excitation involves internal ionization of the crystal of phosphor material (which involves ejection of an electron from the crystal structure), the luminous emission represents a radiative transition which occurs during the recombination of an excited free electron and a hole (i.e. atom of the crystal which has lost an electron). Also, where the excitation raises an electron to a higher energy level without internal ionization, the luminous emission represents return to an energy level near the ground-state energy level. These and other radiative transitions may occur simultaneously in the phosphor material of the display screen such that the luminous emission constitutes the composite of several radiative transitions of excited electronsThese principles of luminescence of phosphor materials are amply described in the literature (see, for example, Luminescence of Solids; H. W. Leverentz; J Wiley & Sons, Inc., New York, 1950; The Physics of Electroluminescent Devices; P. R. Thornton; E. & F. N. Spon, Ltd.; London, 1967, and Electroluminescence; H. K. Henisch; Pergamon Press, Macmillan Co., New York, 1962). Thus, itis believed that the flood electrons from the flood gun excite trapped electrons into the conduction band, as described above, and thereby increase the probability of radiative transitions to lower energy states and, hence, increase the luminous intensity of the signal trace. Where desirable, preexcitation of the phosphor material may be provided by supplying flood electrons prior to a sweep. However, when a transient signal is to be recorded on photographic film, it has been found that slow, postfogging of the film to enhance the signal trace is more effective than prefogging of the film. Thus, combined excitations of the phosphor material by the writing electron beam and by the postexciting flood electrons produce a signal trace on the display screen of much higher intensity luminous output than is possible without the flood electrons and also is more ideally suited for postfogging of photographic film.

The flood electrons may be supplied to the display screen 17 at dissimilar current levels during the period t to 1,. prior to the signal-writing period, as shown in FIG. 2d. Initially, the flood electrons may be supplied at a high current level, as shown in FIG. 211 between times t and after the sweep. Thereafter, the level at which flood electrons are supplied to the display screen 17 may be reduced, as shown between times t and 2 of FIG. 2d, to provide luminous output for slowly postfogging photographic film. The particular levels of flood electrons used and the duration of each level may be selected in accordance with the phosphor material to be postexcited (by the initial level) and in accordance with the response speed of photographic film to be postfogged (primarily by the lower, final level).

The flood gun 13 uses convex spherical electrodes, as shown in FIG. 3, to produce a divergent beam of low energy electrons at intermittent intervals. The flood gun cathode 51 has an electron-emitting surface 53 which is a portion of a sphere having a radius of about 0.50 inches. The cathode may be indirectly heated using a conventional heater filament. The anode electrode 55 of the flood gun 13 includes a spherical mesh 57 which is concentric with the cathode 51 and which has a radius that is greater than the radius of the cathode by about 0.010 to 0.100 inches, or about 0.55 inches in practice. The spherical mesh anode on frame 59 preserves the concentricity of equipotential lines of the radial field and has a mesh size which typically passes greater than 50 percent of the electrons emitted from the cathode. The flood electrons emitted by the cathode are accelerated along divergent paths by this radial field and pass through the anode mesh with substantially uniform current density for uniformly flooding the display screen with electrons over the area of the screen.

The present flood gun is thus a simple diode structure which can be readily controlled using a single biasing potential on the anode with respect to the cathode. Also, the flood electron trajectories initially emerge from the anode on substantially uniformly distributed divergent paths, independent of the particular biasing potential used. This is particularly important in the multilevel postexcitation operating mode of the flood gun according to the present invention, as shown in FIG. 2d, wherein it is desirable to maintain the same divergent beam of flood electrons independent of the cathode-anode biasing potential. The particular divergence angle of the beam of flood electrons is determined by the radii of curvature of anode and cathode surfaces in combination with the radius of the cathode surface and typically may be as high as 35 to 40 for a cathode radius of about 0.155 inches, an anode radius from the common center of about 0.338 inches and an anode aperture of about 0.276 inches.

The spherical surfaces of the cathode and anode may also be approximated in a stepwise fashion using concentrically stacked flat sections of decreasing diameter. Also, where the flood gun electrons of small dimensions are disposed in close proximity an anode having an aperture without a mesh covering in combination with a substantially flat cathode surface may produce approximately a radially divergent electric field that is useful in some flood electron applications.

In operation, the anode electrode 55 may be operated at the potential of the mesh electrode 31,- typically a potential of about 0 to 25 volts. The cathode electrode 51 is coupled to the flood gun controller 47 of FIG. 1 and may be pulsed initially to a potential about 20 volts more negative than the anode electrode to supply flood electrons to the display screen during the time t to t of FIG. 2d and then to a potential about 5-10 volts more negative than the anode during the time t to 1 Operation of the present diode flood gun in this manner assures simple and accurate control over. the energy level of flood electrons supplied by the flood gun 13 for postexciting the phosphor material of the display screen to enhance the luminous intensity of the trace of a high-speed signal and also to postfog photographic film used to record the signal trace.

We claim:

1. Signal display apparatus comprising:

a cathode ray display tube including:

an evacuated envelope having at one end thereof a display screen including phosphor material which emits light in response to electrons applied thereto and having disposed within the envelope near another end thereof a writing electron beam gun for applying electrons directly to the phosphor material of the display screen along a selectable path within the envelope;

a flood electron gun disposed within said envelope spaced away from said path for selectively supplying a flood of electrons directly to the phosphor material'of said display screen in regions supplied with electrons by said writing electron beam gun; and a control means connected to said writing electron beam gun and to said flood electron gun for intermittently activating said guns in a sequential time relationship to supply electrons directly to said regions of the phosphor material of the display screen to activate the phosphor material in said regions of the display screen at a plurality of energy levels as a function of time to increase the light output from said regions.

2. Signal display apparatus as in claim 1 wherein said flood gun is activated to supply electrons directly to the phosphor material of said display screen following the deactivation of said writing gun.

3. Signal display apparatus as in claim 2 wherein said flood gun initially supplies electrons directly to the phosphor material of said display screen at a first energy level and subsequently at a second energy level different from said first energy level during said period.

4. Signal display apparatus as in claim 3 wherein said second energy level at which electrons are supplied to the display screen is lower than the first energy level at which electrons are initially applied to the display screen.

5. Signal display apparatus as in claim 2 wherein said flood gun is activated to supply electrons to the phosphor material of said display screen within the decay period of the luminous output of the phosphor material of the display screen. 

1. Signal display apparatus comprising: a cathode ray display tube including: an evacuated envelope having at one end thereof a display screen including phosphor material which emits light in response to electrons applied thereto and having disposed within the envelope near another end thereof a writing electron beam gun for applying electrons directly to the phosphor material of the display screen along a selectable path within the envelope; a flood electron gun disposed within said envelope spaced away from said path for selectively supplying a flood of electrons directly to the phosphor material of said display screen in regions supplied with electrons by said writing electron beam gun; and control means connected to said writing electron beam gun and to said flood electron gun for intermittently activating said guns in a sequential time relationship to supply electrons directly to said regions of the phosphor material of the display screen to activate the phosphor material in said regions of the display screen at a plurality of energy levels as a function of time to increase the light output from said regions.
 2. Signal display apparatus as in claim 1 wherein said flood gun is activated to supply electrons directly to the phosphor material of said display screen following the deactivation of said writing gun.
 3. Signal display apparatus as in claim 2 wherein said flood gun initially supplies electrons directly to the phosphor material of said display screen at a first energy level and subsequently at a second energy level different from said first energy level during said period.
 4. Signal display apparatus as in claim 3 wherein said second energy level at which electrons are supplied to the display screen is lower than the first energy level at which electrons are initially applied to the display screen.
 5. Signal display apparatus as in claim 2 wherein said flood gun is activated to supply electrons to the phosphor material of said display screen within the decay period of the luminous output of the phosphor material of the display screen. 